Patent Application
20170299056
The invention relates to piston rings, piston assemblies including piston rings, and methods for scrapping oil along an inner surface of a cylinder of an internal combustion engine using piston rings
Pistons for internal combustion engines are designed to contain a plurality of oil rings extending circumferentially around an upper crown for engaging an inner surface of a cylinder wall, and more specifically contact a film of lubricating oil applied to the inner surface of the cylinder wall. The piston rings are designed to seal the combustion chamber, support heat transfer from the piston to cylinder wall, and regulate oil consumption.
One type of conventional piston ring, referred to as a two-piece oil ring, includes two oil scraping rails carved into a solid body. As the piston reciprocates in the cylinder, the two-piece ring twists and thus scraps oil along the cylinder wall. Alternatively, the piston ring could comprise a three-piece assembly. In this case, as the piston reciprocates in the cylinder, the three-piece ring wipes oil along the cylinder wall. Typically, only one of the rails actively controls the oil film. The second rail, which is typically the leading rail in the direction of piston travel, is forced away from the cylinder wall by a combination of hydrodynamic forces, piston tilt, and ring twist. However, the hydrodynamic pressure generated by the oil film ahead of the rails can collapse the ring radially, rendering it less effective. The radial collapse should be counteracted by either the spring action of an expander or by designed-in expansion features in the ring itself. This outward expansion of the piston ring generates a contact pressure of the ring against the cylinder wall. However, an increase in contact pressure corresponds to an undesirable level of oil consumption, which leads to mechanical friction, decreased engine mechanical efficiency, and consequently decreased fuel economy.
One aspect of the invention comprises a piston ring providing improved control of an oil film along an inner surface of cylinder during use in an internal combustion engine. The piston ring is expected to achieve reduced tangential tension, reduced oil consumption, less mechanical friction, increased engine mechanical efficiency, and thus increased fuel economy.
The piston ring includes a body portion and a spring member. The body portion includes an inner surface surrounding a center axis and a running surface facing opposite the center axis. The running surface presents an upper rail and a lower rail each extending radially outwardly relative to the center axis, and the rails are axially spaced from one another. The running surface also presents a fulcrum disposed between the rails. The fulcrum extends radially outwardly relative to the center axis, and an outermost tip of the fulcrum is disposed radially outwardly relative to an outermost surface of the rails. The spring member is disposed along the inner surface of the body portion and surrounds the center axis for pressing the body portion away from the center axis and causing the body portion to pivot about the fulcrum.
Another aspect of the invention provides a piston assembly including a piston ring. The piston assembly comprises an upper crown presenting an upper combustion surface and an outer diameter surface depending from the upper combustion surface and extending circumferentially about a center axis. The outer diameter surface defines at least one ring groove extending circumferentially about the center axis, and each ring groove includes an upper surface and a lower surface facing opposite one another and spaced from one another by a back surface. The piston ring is disposed in at least one of the at least one ring grooves. The piston ring comprises a body portion including an inner surface facing the back surface of the ring groove and a running surface facing opposite the back surface. The running surface of the piston ring extends axially from a top end to a bottom end. The running surface of the piston ring presents an upper rail and a lower rail each extending radially outwardly relative to the center axis and axially spaced from one another. The running surface presents a fulcrum disposed between the rails and extending radially outwardly relative to the center axis, and an outermost tip of the fulcrum is disposed radially outwardly relative to an outermost surface of the rails. The piston ring further includes a spring member disposed along the inner surface of the body portion and surrounding the back surface of the ring groove. The spring member is designed to press the body portion of the piston ring away from the back surface of the ring groove and causes the body portion of the piston ring to pivot about the fulcrum.
Yet another aspect of the invention provides a method for scrapping oil along an inner surface of a cylinder of an internal combustion engine. The method includes providing a piston assembly including a piston ring disposed in a ring groove of an upper crown. The piston ring comprises a body portion including a running surface facing the cylinder liner and an inner surface facing opposite the cylinder liner. The running surface of the piston ring extends axially from a top end to a bottom end. The running surface of the piston ring also presents an upper rail and a lower rail each extending radially outwardly relative to the center axis and axially spaced from one another. The running surface further includes a fulcrum disposed between the rails and extending radially outwardly relative to the center axis. An outermost tip of the fulcrum is disposed radially outwardly relative to an outermost surface of the rails. The piston ring also includes a spring member disposed in the ring groove of the crown portion and along the inner surface of the body portion. The method further includes moving the piston assembly along a layer of oil disposed on an inner surface of the cylinder while the fulcrum of the piston ring engages the layer of oil.
These and other features and advantages of the present invention will be readily appreciated, as the same becomes better understood by reference to the following detailed description when considered in connection with the accompanying drawings wherein:
As shown in
In the example embodiments shown in the Figures, the piston assembly 10 includes an upper crown 18, a pair of pin bosses 20 depending from the upper crown 18, and a pair of skirt panels 22 spacing the pin bosses 20 from one another. The upper crown 18, pin bosses 20, and skirt panels 22 of the piston assembly 10 could have various designs other than the design shown in
The upper crown 18 of the piston assembly 10 includes an upper combustion surface 24 for exposure to a combustion chamber of the engine. The upper crown 18 also includes an outer diameter surface 26 depending from the upper combustion surface 24 and extending circumferentially about a center axis A. The outer diameter surface 26 defines at least one ring groove 28 extending circumferentially about the center axis A, and typically a plurality of the ring grooves 28. Each ring groove 28 includes an upper surface 30 and a lower surface 32 facing opposite one another. The upper surface 30 and the lower surface 32 are spaced from one another by a back surface 34, and the back surface 34 faces away from the center axis A.
As shown in
The piston ring 12 is a two-piece oil scrapper ring and thus includes a body portion 36 and a spring member 38, as shown in
The body portion 36 of the piston ring 12 also includes a running surface 44 facing opposite the back surface 34 of the ring groove 28 and toward the inner wall of the cylinder 16. The running surface 44 extends axially from a top end 46 to a bottom end 48. The running surface 44 also presents an upper rail 50 and a lower rail 52 each extending radially outwardly relative to the center axis A of the piston ring 12. The upper rail 50 and the lower rail 52 are axially spaced from one another, and a third rail, referred to as a fulcrum 54, is disposed between the upper and lower rails 50, 52. The running surface 44 also includes a top land extending from the top end 46 to the upper rail 50, an intermediate land extending from the upper rail 50 to the fulcrum 54 and from the fulcrum 54 to the lower rail 52, and a bottom land extending from the lower rail 52 to the bottom end 48.
In the example embodiments shown in the Figures, the upper rail 50 includes an upper side surface extending radially outwardly from the top land and a lower side surface extending radially outwardly from the intermediate land. The upper side surface and the lower side surface of the upper rail 50 extend toward one another at an angle. The upper rail 50 also includes an outermost surface 56 spacing the upper side surface from the lower side surface, and the outermost surface 56 of the upper rail 50 is flat.
The lower rail 52 of the running surface 44 of the example embodiments also includes an upper side surface extending radially outwardly from the intermediate land and a lower side surface extending radially outwardly from the bottom land. The upper side surface and the lower side surface of the lower rail 52 extend toward one another at an angle. The lower rail 52 also includes an outermost surface 58 spacing the upper side surface from the lower side surface, and the outermost surface 58 of the lower rail 52 is flat.
As shown in the Figures, the fulcrum 54 of the running surface 44, which is disposed between the rails 50. 52, extends radially outwardly relative to the center axis A. The fulcrum 54 extends from the intermediate land to an outermost tip 60, and the outermost tip 60 of the fulcrum 54 is disposed radially outwardly relative to the outermost surfaces 56, 58 of the rails 50, 52. The running surface 44 also presents a convex shape along and adjacent the outermost tip 60 of the fulcrum 54. In the example embodiments, the fulcrum 54 presents a spherical radius along the outermost tip 60. Due to the outward radial location and the convex shape of the fulcrum 54, the body portion 36 of the piston ring 12 is able to pivot in a teeter-tooter-like fashion about the fulcrum 54 and force the rails 50, 52 against the oil film 14 as the piston assembly 10 moves longitudinally along the wall of the cylinder 16 during operation of the internal combustion engine.
In the example embodiment, the fulcrum 54 is centrally located an equal distance from the upper rail 50 and the lower rail 52. However, the fulcrum 54 could be decentralized and thus located closer to the lower rail 52 than the upper rail 50, or closer to the upper rail 50 than the lower rail 52. If the fulcrum 54 is located closer to the lower rail 52, the upper rail 50 will more forcibly interact with the oil film 14. If the fulcrum 54 is located closer to the upper rail 50, contact will preferentially be between the lower rail 52 and the oil firm 14. The fulcrum 54 is expected to act as a third rail to assist control of the oil film 14, in which case lighter tension of the spring member 38 is required to accomplish the desired oil film scrapping. The lighter tension leads to a reduction in the friction loss typically associated with piston rings, which is equivalent to or higher than the first compression ring.
Typically, there is a dimensional relationship between the outer diameter of the fulcrum 54, the inner surface 40 the body portion 36 (also referred to as the ring axial wall), the outer diameter of the rails 50, 52, and the axial clearance between the piston ring 12 and the ring groove 28. In the example embodiments, these dimensional relationships and the tilting angle of the piston assembly 10 as it slides along the cylinder 16 determine the contact angle of the upper rail 50 and lower rail 52. Preferably, the upper rail 50 and the lower rail 52 tip angle will form a convergent obtuse angle for best oil film control.
In the example embodiment, the outermost tip 60 of said fulcrum 54 and the concave groove 42 of the body portion 36 present a first width w1 therebetween. The first width w1 extends perpendicular to the center axis A. In addition, the outermost surfaces 56, 58 of the rails 50, 52 and the concave groove 42 of the body portion 36 present a second width w2 therebetween. The second width w2 also extends perpendicular to the center axis A. Also in the example embodiments, the top end 46 the running surface 44 of the piston ring 12 and the upper surface 30 of the ring groove 28 present a first clearance distance D1 therebetween. The bottom end 48 of the running surface 44 of the piston ring 12 and the lower surface 32 of the ring groove 28 present a second clearance distance D2 therebetween. The fulcrum 54 can also include an oil slot 62 for draining the lubricating oil, if desired.
To enhance boundary lubrication, the piston ring 12 typically includes a coating applied to the running surface 44 and continuously from the top end 46 to the bottom end 48 of the running surface 44. The coating is applied along each of the rails 50, 52 and the fulcrum 54. The composition of the coating is typically a premium coating selected from a chromium-based coating, a chromium-ceramic coating, a nitrided coating, a DLC coating, and a PVD coating. The DLC and PVD coatings are especially beneficial to reduce scuffing and durability.
As the piston assembly 10 slides along the cylinder 16, the spring member 38 presses the body portion 36 of the piston ring 12 against the inner surface of the cylinder 16, and more specifically against the oil film 14 applied to the inner surface of the cylinder 16. The fulcrum 54 is expected to always be either in boundary lubrication film contact with the inner surface of the cylinder 16 or ride the thin hydrodynamic oil film 14. Thus, during upward or downward movement of the piston stroke, the piston ring 12 will present, to some extent, three oil controlling features in succession. Consequently, for the same tip pressure, reduced oil consumption should be observed. Or, for the same oil consumption, a lower tangential tension of the piston ring 12 can be used, which in turn reduces oil consumption.
The spring member 38 of the piston ring 12 is disposed along the inner surface 40 of the body portion 36 and surrounds the back surface 34 of the ring groove 28. The spring member 38 is placed under a tangential tension to press the body portion 36 of the piston ring 12 away from the back surface 34 of the ring groove 28 and cause the body portion 36 of the piston ring 12 to pivot about the fulcrum 54. In the example embodiment, the spring member 38 is a helical spring, but another type of spring could be used.
Obviously, many modifications and variations of the present invention are possible in light of the above teachings and may be practiced otherwise than as specifically described while within the scope of the appended claims.
